Mani is a Mexican man who appears in the Alta neighborhood, near the corner of Elgin and Hawick Avenues, by a gazebo in a courtyard filled with food shacks and bordered by two Mexican restaurants. When approached, he speaks to the player and surrounding passers-by entirely in Spanish, though all of his words are translated into English via subtitles. He talks primarily about his frustration with being ignored by those around him and the threats he receives in Los Santos for being Mexican. He's also desperate to get a job as an actor to the point to reach for blowjobs.
Mani will be terrified of Trevor, to the point of bowing on the floor to him.
Though Mani is dressed as a mariachi, he identifies himself as an actor. There are mariachis in the game, which are featured in the mission The Civil Border Patrol. Coincidentally, Trevor is involved with the harassment of the mariachis in that side mission, which may explain why Mani acts terrified upon encountering Trevor himself.